Controls which display relatively low power dissipation are now commonly available to regulate power delivered by an AC line source to a load. For example, in connection with an incandescent lighting element, it is common to series connect the lighting element with a triac, a device which requires triggering in each half cycle of the AC line voltage for conduction of load current. The required triggering signal may be generated directly from the AC line voltage by a resistive voltage divider, and may be applied to the triac through a device with a predetermined turn-on voltage, such as a diac, to ensure predictable triggering. One element of the resistive divider may be a variable resistor whose resistance value can be manually adjusted by a user to regulate triggering and ultimately the amount of power delivered by the device to the load.
This arrangement and similar variants thereof are commonly referred to as "phase angle control". In essence, the resistive divider and threshold turn-on device suppress triggering of the triac and conduction of load current until the AC voltage has achieved a certain phase angle relative to the most recent zero crossover of the AC voltage. One significant limitation associated with prior art devices is that phase angle can only be regulated in the range of 0 to 90 degrees in each half-cycle regardless what ultimate voltage divider ratio is obtained. In the context of lighting control, power consumption cannot be conveniently reduced to less than about one half of that otherwise expected under full line voltage. In the operation of motors, such controls permit speed control only over a very specific range, control over very low speed operation being unavailable. With certain prior devices which use RC timing circuits to generate a triggering signal, there is the additional disadvantage that operation is dependent on line voltage and frequency. Accordingly, a power controller appropriate for operation with 110 volt AC line sources at 60 hertz may not operate properly in response to 220 volt AC line source or to one involving a different frequency.
It is an object of the present invention in preferred form to provide an AC power controller which overcomes the various shortcomings associated with the prior art devices.